It doesn't help to tell a person, child or adult, that they don't need to be afraid of something, or that it won't hurt him, especially if it can indeed hurt him. Acting casual around bees is good, telling the bee to go away is probably ineffective unless the bee magically goes away each time you tell it to. Educating about bees is great, as long as seeing the pictures doesn't send him into a frenzy. If he can tolerate them, even videos might help. I was deathly afraid of bees for several years, but gradually got over my fear by watching them and realizing they hardly registered my presence.
If he's verbal enough, you might have this chat a few times over the next few months: "You really feel frightened when you see bees, don't you, sweetie? (yup!) I understand. Do you want to tell me about it? (nope.) Hmm. I wonder if you're afraid they will sting you? (yup!) You know, lots of people are afraid of that. Even some big people. (oh!) Yep, those little bees seem awfully powerful, if they can scare even grownups, huh? (yup!) Well, I wonder if you know why bees can sting? (???) It's because they are so small, and they make delicious honey that great big animals like to eat. Big, strong animals like bears LOVE honey, and they will wreck the bees' home just to take the honey. Did you know that? (!!!) So the little tiny bees had to come up with a way to protect their homes. They found out they could sting to keep the bears away from their honey. (!!!) Are you planning to steal honey out of a bee's home? (not too likely) Well, I don't think the bees will be interested in you unless you are bothering their home. That's what their stingers are for, protecting their honey. (hmmm – skeptically). So it's okay to be afraid of bees, but they probably won't pay any attention to you unless you're trying to take honey from their home."
If that doesn't gradually lessen his anxiety, you can try the following: "So, being afraid of bees is really spoiling a lot of your fun outside, isn't it? (well, sometimes) Yeah, I'm a little sad when I see you so afraid of bees that you can't even play. <don't mention the screaming> I wonder if there's anything that could help you worry about them less. Do you have any ideas? (maybe, …) Okay, I'll be happy to help you try that. And here's another idea. What if we just watch the bees on the flowers from inside this window / from across the back yard for a little while every day? You might notice that they're not paying a bit of attention to you. Do you think that might help you be less afraid?"
Validate his fear without feeding it, empathize with how difficult it is to be scared, let him know he can talk to you and ask you anything. He may need a year or two, but he should eventually be okay. Tolerant, if not friendly.
Also, look into orchard mason bees, a native variety that can almost never be provoked to sting (and if they do, it's supposed to be a very minor discomfort). You can get inexpensive nesting tubes for them – I have several – and watch them hatch, mate, and build a new season's nest each April/May. They do not make honey. By midsummer, their life cycle is finished, and you wait until next spring to see the next batch. Cute little things!